The Overexpression of AmpC is Not Required but May Contribute to Imipenem/Relebactam Non-susceptibility

In the US, carbapenem resistance in Pseudomonas aeruginosa is strongly linked to the regulation of chromosomal resistance determinants, AmpC and OprD. The {beta}-lactamase AmpC requires overexpression and genetic modifications to be capable of inhibiting imipenem activity. The outer membrane porin OprD can be downregulated or undergo genetic modifications that strongly correlate with imipenem non-susceptibility. Co-administration of imipenem and the {beta}-lactamase inhibitor, relebactam, can lower imipenem MICs and restore susceptibility. However, how this occurs in P. aeruginosa isolates that do not overproduce AmpC or produce a functional OprD for imipenem entry is not understood. Therefore, we investigated whether imipenem could enter P. aeruginosa in the absence of OprD and whether any of the chromosomal {beta}-lactamases (AmpC, OXA-51, PIB-1) contributed to imipenem and/or imipenem/relebactam non-susceptibility. This investigation evaluated 17 imipenem non-susceptible clinical isolates and 3 laboratory strains of PAO1, two of which were porin deletion mutants for either oprD or opdP. Expression of OXA-50 and PIB-1 RNA was similar to PAO1. However, all 20 isolates exhibited AmpC induction under sub-lethal exposure to imipenem. This occurred in the absence of detectable OprD protein in 18 isolates. Collectively, our data identify that OprD is not the only channel required for imipenem entry and that in many isolates the restored susceptibility to imipenem by imipenem/relebactam was due to the interaction of relebactam on the overexpression of AmpC due to imipenem induction.